For over 50 years, the cathode ray tube (CRT) has been the principal device for displaying visual information. Although the CRT provides remarkable display quality in terms of brightness, color, contrast and resolution, it is large, bulky and power hungry. It is not a technology that can be portable and easily scaled to large sizes (50xe2x80x3 diagonal or larger). Several display technologies are in development or matured to manufacturing that try to fill this void.
As one of these technologies, field emission displays (FEDs) have been under development for several years now. They have the promise of providing CRT-like image quality in a thin, compact and lightweight form. FEDs rely on cold cathode technology as the source of electrons that are controlled and accelerated to the phosphor-coated faceplate. The impact of the electrons on the phosphor creates the light that is used to form the image. Different phosphors are used to create the red, green and blue colors, as in a CRT.
The cold cathodes used in FEDs vary from arrays of semiconductor or metal microtips, coatings of a variety of carbon films on microtip arrays or on flat surfaces, and coatings of wide-bandgap materials. The carbon films span a complete range of materials from diamond or diamond-like coatings, graphitic, amorphous, Amorphic(trademark), carbon nanotubes and other fullerene carbon phases, and mixtures of any and all of these phases. Other cold cathode technologies are microtips structures with a coating of carbon or other materials to lower work function, to harden the tip, or sharpen the tip. The disclosure described herein is relevant to any and all of these cold cathode technologies.
Most of the microtip technologies have developed such that the field that is used to extract electrons from the tips comes from the electrical potential difference between a gate electrode placed around the tips and the tips themselves. FIG. 1 shows the prior art in microtip technology. Typically, the gate 11A, 11B, 11C is built and integrated onto the same substrate 12A, 12B, 12C as is used to support the microtips 13. One problem with this and other cold cathode technologies has been to control the current emitted from the cathode. In microtip technologies, this is done by electrically connecting the microtips or arrays of microtips to the electrical bus lines that define the rows or columns in the display through a passive resistor or an active circuit containing diodes, capacitors, and transistors. FIGS. 2 and 3 are examples of prior art. In both examples, circuits on the cathode that link directly to the tip control the current emitted from the tip. For example in FIG. 2, transistors on the substrate at each pixel switch the current to the microtip array. In FIG. 3, the active circuits are external to the display panel, but still perform the same function of controlling the microtip emission current through circuits linked directly to the microtips. In these examples the gate is either common to all pixels in the display or the gate electrode is separated into rows and the each gate row is common to all pixels in that row, and the active elements that control the emission current control the tip electrode and not the gate electrode. Although this approach may work well for microtips, for other cold cathode technologies it may be impractical.
Many of the carbon film cold cathode approaches require high temperature to grow or fabricate the carbon layer. This means that the substrate must be able to withstand high growth temperatures, above the point at which glass is not a suitable choice. In other cases, glass or other insulating substrates may not be suitable since for certain carbon film growth techniques, such as plasma enhanced DC-CVD, a conducting substrate is needed, or at the very minimum, a conducting layer on the insulating substrate. High temperature glass or ceramic substrates are expensive and break easily when subjected to thermal gradients. One choice of substrate material on which to grow carbon films is steel sheets, such as 304 stainless steel or stainless alloys such as 42-6 (a stainless alloy containing 42% Ni, 6% Cr). Stainless sheets are relatively inexpensive. One can purchase highly polished 304 stainless plates for $4.00 a square foot or less, and it is readily available since it is used commercially to cover walls of buildings and build metal furniture. Steel substrates are strong, handle thermal stress much better then glass, and are impervious to air so they can hold a vacuum like glass.
The problem with putting a cathode material on a conducting substrate such as silicon (Si) or metal is that it is difficult to electrically isolate the pixel areas and the electrical buslines connecting and controlling the pixel areas. One can deposit insulating layers on top of the conducting substrate, but this may again interfere with certain carbon layer growth techniques. Furthermore, even with an isolated layer between the buslines and the conducting plate, the parasitic capacitance between the buslines and the conducting ground plane would cause excessive power dissipation during display operation as elements are being constantly and rapidly electrically switched from one state to another.
Another problem is that multilayer structures do not survive well in the high temperature growth processes performed in carbon-rich atmospheres. Adhesion of different layers becomes more difficult at higher temperatures because of stresses developed in the different layers as a result of differences in thermal expansion. Furthermore, carbon layers or fibers can easily grow across edges of insulating films and thus electrically short conducting layers together. Thus, a solution is required to overcome these difficulties.